Cellular wireless is an increasingly popular means of personal communication in the modern world. People are using cellular wireless networks for the exchange of voice and data over cellular telephones, Personal Digital Assistants (PDAs), cellular modems, and other devices. In principle, a user can seek information over the Internet or call anyone over the Public Switched Telephone Network (PSTN) from any place inside the coverage area of the cellular wireless network.
In a typical cellular wireless system, an area is divided geographically into a number of cell sites provided by a radio access network (RAN). The RAN typically comprises one or more base transceiver stations (BTSs), each of which has one or more antennas that radiate to define a radio frequency (RF) radiation pattern. The BTS(s) of the RAN may then be coupled with a base station controller (BSC) or radio network controller (RNC), which may in turn be coupled with a telecommunications switch or gateway, such as a mobile switching center (MSC) or packet data serving node (PDSN) for instance. The switch or gateway may then provide connectivity with a transport network, such as the public switched telephone network (PSTN) or the Internet for instance.
When a mobile station (such as a cellular telephone, a wirelessly equipped PDA or personal computer, or another suitably equipped device) is positioned in a cell, the mobile station communicates via an RF air interface with the BTS of the cell. Consequently, a communication can be established between the mobile station and another entity, via the air interface and the RAN.
With the explosive growth in demand for wireless communications, the level of call traffic in most cell sites has increased drastically over recent years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors (which can be visualized ideally as pie pieces), each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae. As such, each sector has an azimuth, which is understood to be the general direction of radiation of the sector, such as the direction the antenna of the sector is pointing.
In a Code Division Multiple Access (CDMA) wireless network and perhaps in other types of networks, each cell employs one or more carrier frequencies, and each sector is distinguished from adjacent sectors by a pseudo-random number offset (PN offset). Further, each sector may concurrently communicate on multiple different channels, distinguished by “Walsh codes”. When a mobile station operates in a given sector, communications between the mobile station and the BTS of the sector are carried on a given frequency and are encoded by the sector's PN offset and, perhaps, a given Walsh code.
According to well known industry standards, a mobile station can communicate with a number of “active” sectors at a time. Depending on the system, the number of active sectors may be up to three or six, for instance. The mobile station receives largely the same signal from each of the active sectors and, on a frame-by-frame basis, may select the best signal to use.
A mobile station maintains in its memory a list of the sectors in its “active” set. In addition, it maintains in its memory a list of “candidate” sectors (e.g., up to six), which are those sectors that are not yet in the active set but that have sufficient signal strength that the mobile station could demodulate signals from those sectors. Further, the mobile station maintains a list of “neighbor” sectors, which are those sectors not in the active set or candidate set but are in close vicinity to the mobile station. All other possible sectors are members of a “remaining” set.
In existing systems, to facilitate a determination of which sectors should be in the mobile station's active set, all base stations emit a pilot channel signal on each sector, typically at a power level higher than other downlink signals. A mobile station then constantly measures the strength (Ec/Io, i.e., energy versus spectral density) of each pilot that it receives and notifies the RAN (e.g., a BSC serving the mobile station) when pilot strength falls above or below designated thresholds. The RAN, in turn, provides the mobile station with an updated list of active pilots.
In one arrangement, for instance, the RAN may initially transmit to the mobile station (e.g., over a downlink control channel or traffic channel) a Handoff Direction Message (HDM), containing parameters such as (i) the PN offsets of the sectors in the active set and (ii) the following handoff parameters that relate to pilot signal strength:                T_ADD: Threshold pilot strength for addition to the active set (e.g., −14 dB)        T_COMP: Difference in signal strength from an active set pilot (e.g., 2 dB)        T_DROP: Threshold pilot strength for removal from the active set (e.g., −16 dB)        T_TDROP: Time for which an active set pilot falls below T_DROP to justify removal from the active set (e.g., 2 seconds)Additionally, the RAN may initially provide the mobile station with a Neighbor List Update Message (NLUM), which identifies the “neighbor” sectors for the current active set.        
The mobile station may then monitor all of the pilot signals that it receives, and the mobile station may determine if any neighbor pilot exceeds T_ADD by T_COMP. If so, the mobile station may add the pilot to its “candidate” set and send a Pilot Strength Measurement Message (PSMM) to the base station, indicating the estimated Ec/Io for the pilot, with the pilot designated by PN offset. Depending on current capacity and other issues, the RAN may then agree to allow the mobile station to hand off to the designated sector. Accordingly, the RAN may reserve a channel resource (such as a Walsh code) in the sector and may send to the mobile station (e.g., via each of the mobile station's current active set sectors) an HDM listing the pilot as a new member of the mobile station's active set and directing the mobile station to use the reserved channel resource in the added sector. Further, the RAN may send to the mobile station a new NLUM, designating a new neighbor list corresponding to the mobile station's revised active set.
Upon receipt of the HDM, the mobile station would then add the pilot to its active set as instructed, and the mobile station would send a Handoff Completion Message (HCM) to the RAN, acknowledging the instruction, and providing a list of the pilots (PN offsets) in its active set, thereby completing the handoff.
Similarly, if the mobile station detects that the signal strength of a pilot in its active set drops below T_DROP, the mobile station may start a handoff drop timer. If T_TDROP passes, the mobile station may then send a PSMM to the RAN, indicating the Ec/Io and drop timer, and similarly designating the pilot by PN offset. The RAN may then respond by sending an HDM to the mobile station, without the pilot in the active set. And the mobile station may then receive the HDM and responsively move the pilot to its neighbor set and send an HCM to the RAN. Further, the base station may likewise send a new NLUM to the mobile station to update the mobile station's neighbor list.
To facilitate handoff of a mobile station from one sector to another, nearby sectors of a cellular system should have distinct identifiers that the mobile station and RAN can reference when engaging in handoff signaling. In a CDMA system, for instance, nearby sectors have distinct PN offsets, and the PSMM, HDM, and HCM signals reference to those PN offsets. In other types of cellular systems, other types of sector identifiers may be used.
Provided with such distinct sector-identifiers, when a mobile station requests handoff to a sector designated by a particular identifier, the RAN can readily determine which sector the mobile station means, and if appropriate the RAN can reserve resources in that sector and direct the mobile station to hand off to the sector. Accordingly, it is generally important that all of the sectors to which a mobile station can hand off from the current serving sector(s) have different identifiers from each other.
If two or more sectors were to have the same identifier as each other and are near enough to each other, the identifier could give rise to ambiguity. In particular, if a mobile station detects a strong pilot from one such sector and requests handoff by reference to the sector identifier, the RAN would not know which sector the mobile station means. In a worst case, the RAN could assume the mobile station means a particular one of the sectors and could reserve resources in that sector, but the pilot actually received by the mobile station could have been in another sector altogether. In that case, when the RAN grants the mobile station's handoff request, the mobile station may be unable to communicate on the sector, and the mobile station may experience a dropped call, which could result in a poor user experience.
On the other hand, cellular systems are typically designed to re-use sector identifiers or other parameters for sectors that are deemed to be located far enough apart from each other that such ambiguity would not arise. One reason to re-use sector identifiers in this manner is to conserve a potentially limited supply of sector identifier values. For instance, a CDMA system typically uses a limited number of PN offsets, so that mobile stations will only have to scan the airwaves for those particular PN offsets. Consequently, in a typical CDMA system, sectors that are deemed to be located far enough apart from each other may have the same PN offset as each other, while sectors that are located closed to each other should not have the same PN offset as each other.
Considering this ambiguity problem, a CDMA RAN is also typically arranged to allow a mobile station to hand off only to sectors that are listed in the mobile station's current neighbor list, since those sectors are assumed to be close enough to the mobile station's current serving sector(s), and since, by design, those sectors would not have the same identifiers as each other. In practice, a mobile station may detect a strong pilot from a sector not in the mobile station's neighbor list, and the mobile station may report it to the RAN. However, upon receipt of such a request, a CDMA RAN would not direct a handoff to the indicated sector, because the sector is not a member of the mobile station's current neighbor list. Rather, the RAN may simply record the pilot measurement for later engineering analysis or for other purposes.